Recently, interests on energy storage technologies are more increased. As the energy storage technologies are extended to cellular phones, camcorders and notebook PC, and further to electric vehicles, the demand for high-energy concentration of a battery used as a power source of such an electronic device is increased. A lithium ion secondary battery is one of the most satisfactory batteries, and many studies are now in active progress.
Among secondary batteries currently used, a lithium secondary battery developed in the early 1990's includes an anode made of carbon material capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxide, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent.
The lithium secondary battery has an average discharge voltage of about 3.6 to 3.7V, which is advantageously higher than those of other batteries such as alkali batteries or nickel-cadmium batteries. To give such a high operation voltage, an electrolyte composition electrochemically stable in a charging/discharging voltage range from 0 to 4.5V is required. For this purpose, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate or propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate are suitably mixed is used as a solvent of electrolyte. A solute of electrolyte commonly uses a lithium salt such as LiPF6, LiBF4, LiClO4 and LiN(CF3SO2)2, which serves as a source for supplying lithium ions in a battery and thus enables the lithium battery to operate.
Such a lithium secondary battery demands various characteristics depending on its usage. For example, a high-power battery used as an auxiliary power source of a hybrid vehicle should have high power density and excellent cycle and high temperature charcteristics, and this battery should also have high low-temperature power for helping the vehicle to start at a low temperature.
In this consideration, vinylene carbonate (VC) is used for improving high temperature cycle performance of a lithium secondary battery. However, vinylene carbonate deteriorates low temperature power performance of the battery. Meanwhile, ethylene sulfate is known to attribute to improving low temperature characteristics of a lithium ion battery.
As mentioned above, various kinds of additives have been proposed to improve characteristics of batteries, but there is no non-aqueous electrolyte solution capable of both overall high temperature and low temperature characteristics of batteries.